The present invention relates to an electrical control apparatus which includes means for setting and indicating two types of parameters such as process and control parameters in which a common indicating means and a common control are utilized for displaying and setting the two different types of parameters.
A prior art control apparatus is described in the Barber Colman Company instruction manual dated September 1979 for "1253 IN 100-3 560 Series Microprocessor Controller For Process Heating Applications." As described in the instruction manual, the controller displays the actual value of a process parameter or variable (e.g. voltage or current corresponding to a measured temperature) and provides a controlled output which is related to the process parameter by the control function of the controller. A common indicator and a common control on the control panel are used to display and set the process parameter and also a control parameter (e.g. a tuning constant) in different operating modes of the apparatus. The value of the process parameter can be set as a limit value or an alarm value, etc., relative to the actual monitored or controlled value of the process parameter. The control panel includes an access door which can be swung open to expose a mode control switch for switching the apparatus from a normal control mode of operation to a set-up mode of operation. Upon actuation, the mode control switch conditions the apparatus to operate in the set-up mode in which a control parameter is displayed and can be set. The value of a selected control parameter can be set by operating the common control on the control panel which incrementally changes the value and indication of the selected control parameter when the apparatus is in the set-up mode. The common control can also be used to set a value of a process parameter when the apparatus is in the normal control mode of operation with the access door swung closed to cover the mode control switch. The apparatus can however also be operated in the normal mode with the access door swung open and even after the mode control switch has been actuated depending on the state of a sequence involving actuation of the mode control switch. It is therefore possible to operate the apparatus in either operating mode with the access door swung open and this can lead to the inadvertent setting or changing of the wrong parameter.